Hot forming tool

ABSTRACT

Disclosed is a hot forming tool ( 1 ) which consists of a main tool member ( 2 ) having an at least partial surface coating ( 4 ) and which can be obtained by providing the main tool member with a raised metal relief that is then entirely or partly oxidized and converted into a protective layer.

FIELD OF THE INVENTION

The invention is in the field of manufacture of tubular metal workpiecesand relates to forming tools, especially piercing plugs, forgingmandrels and rolling mandrels with improved stability.

STATE OF THE ART

Seamless steel tubes are generally made in three forming stages onrespective rolling mills by hot forming. In a first stage, on aso-called cross-roll piercing mill a solid steel block heated to about1200° C. is pierced by means of an internal tool, the piercing plug,into the hollow bloom. Here, the block is driven by means of inclinedrolls over the piercing plug. In the second forming stage, in alongitudinal rolling process the hollow bloom is reduced over the innertool, a rolling mandrel, in diameter and wall thickness and stretched inthe longitudinal direction. In the third forming stage, the rollingstock is converted to the required dimensions in diameter and wallthickness, where usually no internal tool is used.

The inner tools in the first two forming stages are exposed in theproduction to high temperatures and mechanical pressures. In most cases,the inner tools are made of heat resisting steel. In the production,especially with larger rolling times a successive heating of theinternal tool is often unavoidable. Due to heating the strength of thetool decreases and the tool can no longer withstand the mechanicalstresses. The tool deforms and breaks.

To achieve a long service life, piercing plugs are provided with naturalscale layers. These scale layers inhibit the flow of heat from theworkpiece to be shaped into the tool and protect the tool from rapidheating and rapid loss of strength. When forming higher alloyedmaterials, the scale layer is removed, however, quickly and the thermalprotection fails.

In rolling mandrels scaled tools or tools provided with a chromium layerare used depending on the forming process course. Corresponding piercingplugs are known from German Patent Application DE10 2008 056 988 A1 (SMSMEER). A disadvantage, however, is that the thermal insulation againstthe heat flow from the workpiece to be shaped into the tool is low.Thus, particularly with internal tools, which are used at reduced speedand length of contact, heating of the inner tool and its failure due todeformation and fracture occur.

The tool life could be improved if the thickness of the oxide layercould be enlarged. Then, the thermal insulation is better and duringabrasive wear the protective layer would remain preserved longer.

The protective layer is formed naturally from the base material byconversion to iron oxides, but does not have high stability. It isbrittle and porous and therefore can be easily destroyed by mechanicaland thermal stress. Therefore, these protective layers are limited inthickness. The limit of the layer is about 0.8 mm. The protective effectof such a layer is therefore limited accordingly. Heat penetratestherefore into the main body of the tool and reduces its strength,whereby it then comes to premature failure of the tool. Withhigh-alloyed work-pieces abrasion leads relatively quickly, i.e. by asmall length of the rolled material to the removal of the protectivelayer.

From the international patent application WO2011 107214 A1 (SMS MEER)piercing plugs or rolling mandrels are known for the production ofseamless tubes or forging mandrels for hot forging of tubular workpiecesof metal which have a surface profiling, in which the oxide layer isapplied. In this way, better adhesion and longer service life to beachieved.

Similar tools, in which the coating consists of molybdenum, are knownfrom EP0385439A1 (NKK CORP.).

Subject of the European patent application EP 2404680 A1 (SUMITOMO) isthe manufacture of steel tubes according to the Mannesmann process.Claimed is a piercing plug the feature of which is, that it inhibits achannel, through which a lubricant is lead during piercing. According to[0053] the piercing plug can be coated by iron. For this an iron wire islead to a spraying device where it is molten. The molted iron is thensprayed onto the piercing plug, i.e. a continuous coating is developing.

In practice, the preparation of such profiled tools, however, proves tobe expensive since the profiles have to be individually cut into thepiercing plugs, and also lead to material losses. The manufacturing costof a profile disproportionately increase with the size of the introducedgrooves. An economic and feasibility limit is reached at just a fewmillimeters. Another disadvantage of the profile cut into the base bodyis the limitation of the material on good oxidizable steels. These havein particular a low chromium content and thus low hardness.

The object of the present invention has therefore been to provide hotforming tools with improved stability, which are free from theabove-stated disadvantages. In particular, these tools should have anoxide layer having a higher strength compared to the prior art, whichcan be also applied easily and without loss of material.

DESCRIPTION OF THE INVENTION

A first aspect of the invention relates to a hot forming tool comprisinga tool body having at least pro rata surface coating, which can beobtained, by that the basic body is provided with a raised metallicrelief, which is subsequently completely or partially oxidized andconverted into a protective layer.

Under ‘raised’ has to be understood, that the relief is raising relatedto the surface of the tool (‘hill structure’) and thus is contradictoryto a profiling where profile grooves are carved into the surface(‘valley structure’).

Another aspect of the invention includes a method for producing a hotforming tool comprising a tool body having at least pro rata surfacecoating, in which

-   (a) the body is provided with a raised metallic relief, and-   (b) successively the metallic relief is completely or partially    oxidized and converted into a protective layer.

The application of raised reliefs is the reverse case to a profiling ofthe tool. For the purposes of the invention, therefore, material isadded and not removed. Surprisingly, it was found that the reliefformation is in contrast to the profiling not only much easier torealize, but by complete or partial conversion of the relief material,even a considerably harder and thus more stable oxide film is obtained,which leads to a significant improvement in tool life. The inventionalso provides the possibility of selecting the relief material to varythe quality of surface protection and adjust the process conditions.

The economic benefit of the invention is obvious and is in particular inthe reduction of tool costs during the production of steel products, aswell as the extension of the rolling time, which is usually associatedwith larger lengths of the rolling stock and reduced material waste.

Tools

Hot forming tools of the present invention are preferably a piercingplug or a forging mandrel, which are typically made of steel. Theinvention includes under this preamble, however, in principle, any othermetallic workpiece, in which the body is to be protected against heatinflux. The term metal is not limited to iron and steel, but alsoincludes other metallic materials including metal-composite materialsthat are to be supplied to a hot forming.

But not only in piercing plugs, the inner tools in piercing by crossrolling, with the other inner tools that are used in the production ofseamless steel tubes, the surface coating of the invention may beadvantageously employed. In the rolling mandrels, the inner tools in therolling mills with several successively arranged roll stands in thesecond forming stage is particularly important to ensure that thefriction between tool and rolling stock is low. Therefore, the surfacelayer of the invention has to be grinded and polished for thisapplication. Also, an additional layer can be applied, made of chrome onthe protective layer according to the invention.

The raised relief, which is applied to the base body can be pronouncedquite differently, the alternative embodiments are all suitable inprinciple to fulfill the task in full.

In a first embodiment, it may be simply a wrapping of the body with awire, preferably a steel wire at the raised relief.

In a second embodiment, the raised relief can represent a metal fabricor metal mesh, which is applied to the base body.

The metallic bodies applied to the surface of the tool are preferablymade from a steel mesh, for example with a steel wire thickness of about1 to about 5 mm and preferably about 1.5 mm and a mesh spacing of about1 to 5 mm and in particular about 2.5 mm. Under the mesh spacing thedistance of the center lines of two adjacent fabric elements is to beunderstood.

In a third embodiment the raised relief may be an irregular coating, asis achieved by chemical or physical deposition of metal from the vaporphase.

Relief Formation

The application of the raised reliefs can by very different—simple andcomplex—procedures, which yet solve all the object of the invention tothe full extent.

In a first embodiment, the base body is simply wrapped with a wire,preferably a metal wire.

In a second alternative embodiment, a metal fabric or a metal mesh isused instead of the wire. This may be preformed, for example by formingthe shape of the tool and then mounted on the base body. In order toincrease the strength, it is advisable to weld the wire coil or themetal fabric to the base body.

In a third alternative embodiment, it is possible to produce the reliefon the surface of the base body by chemical or physical vapor deposition(Chemical/Physical Vapor Phase Deposition, CVD/PVD).

The term chemical vapor deposition is a group of coating methods whichare used inter alia in the manufacture of microelectronic components andoptical waveguides. At the heated surface of a substrate, a solidcomponent is separated due to a chemical reaction from the gas phase. Aprerequisite is that volatile compounds of the component layers exist,the entrained solid layer at a given reaction temperature. The method ofthe chemical vapor deposition is characterized by at least one reactionon the surface of the workpiece to be coated. This reaction must be atleast a gaseous starting compound (starting material) and at least tworeaction products—to be involved—of which at least one in the solidphase. To over competing vapor phase reactions to promote thosereactions at the surface and thus to avoid the formation of solidparticles, the process is preferably carried out at reduced pressure.

Unlike the CVD, the starting material is converted into the gas phaseusing the preferred PVD. The gaseous material is then led to thesubstrate to be coated, where it condenses and forms the target layer.Examples are classical evaporation processes, such as thermalevaporation, electron beam (electron beam evaporation) or laser beamevaporation (pulsed laser deposition). For the purposes of the presentinvention, preferred is sputtering in which the starting material issputtered by ion bombardment and transferred into the gas phase fromwhich it can then be deposited on the basic body. All these processeshave in common that the material to be deposited is in solid form in themostly evacuated coating chamber. By bombardment with laser beams,magnetically deflected ions or electrons as well as by arc discharge,the target is evaporated. The proportion of atoms, ions or largerclusters in the vapor varies from procedure to procedure. The vaporizedmaterial moves either ballistically or performed by electric fieldsthrough the chamber and impinges on the parts to be coated, where itcomes to the layer formation.

For achieving that the vapor particles reach the components, and are notlost by scattering at the gas particles, the work must be done invacuum. Typical operating pressures are in the range of 10 ⁻⁴ Pa to 10Pa. Since the vapor particles propagate straight, areas that are notvisible seen from the steam source, are coated with a lower depositionrate. In order to produce a relief and no homogeneous coating a rotationof the substrate will be omitted different from the usual procedure.

A fourth alternative embodiment of the relief formation comprises theso-called thermal spraying. Here additional materials, the so-calledspray additives are melted off, at or on inside or outside a sprayburner, accelerated in a gas stream in the form of spray particles andthrown on the surface of the component to be coated. The componentsurface in this case (in contrast to the cladding) is not melted andthermally loaded only slightly. A layer formation takes place, as thespray particles are flattening more or less depending on process andmaterial when impinging on the component surface, stick primarily bymechanical bonding and layer by layer to build the spray layer. Qualitycharacteristics of spray coatings are low porosity, easy bonding to thecomponent, avoidance of cracks and homogeneous microstructure. The layerproperties obtained are significantly influenced by the temperature andthe speed of the spray particles at the time of incidence to the surfaceto be coated. The surface state (purity, activation temperature) alsoexerts a significant influence on quality characteristics such asadhesion.

As energy carrier for the melting of the spray additive material areused electric arc (arc spraying), plasma jet (plasma spraying),fuel-oxygen flame or fuel-oxygen high-speed flame (conventional andhigh-velocity flame spraying), fast, preheated gas (cold gas spraying)and laser beam (laser beam spraying). According to EN 657 DIN standardspray methods are classified according to these criteria.

Using this method, the base body may be coated not only with metals butalso oxide-ceramic materials and carbide materials (or in generalcomposites). Preferably in this embodiment the coating takes place witha steel/ceramic mixture.

While the base body is preferably made of steel, it is valid for thematerial forming the raised relief, the requirement that it is at leastcapable rata for forming an oxide layer. Preferably, this is iron orsteel so that a layer of iron oxide, preferably scale is generated. Itcan be used as said, a mixture of iron/steel and ceramics for example ina weight ratio of about 20:80 to about 80:20.

It is understood that the relief may have different forms, ranging fromregular (round, square, etc.) to any freeform. It can also be usedcomposite materials, i.e. for example, a molybdenum fabric that isapplied to the steel body. The fabric element can also consist of acomposite of hard chrome steel (inside) and well oxidizable steel(outside). As the spacer, also combustible materials may be employed. Itis also possible, for better heat insulation to embed ceramic.

Oxidation

The complete or partial conversion of the metallic reliefs in an oxidicprotective layer may be produced by known methods of the prior art, forexample by flame spraying, plasma spraying, or is carried out by athermochemical process.

In the oxidation of the tool with the metallic body applied on itssurface, for example a steel fabric, a part of the surface of the toolbase body and a part of the relief deposited on the surface is convertedinto oxide. At the same time, an additional oxide layer is formed on allsurfaces, typically to about 3000 microns, and especially about 1.500 toabout 2,500 microns. Thus, also oxide is formed in the spaces betweenthe bodies, for example, between the tool body and the applied steelfabric and within the meshes of the steel fabric. The result is aparticularly thick protection layer which is reinforced by means of aninternal body. In particular, the layer thickness is different than inthe production of grooves not limited to a few millimeters. Layerthicknesses of 10 mm and more can be produced without difficulty and atlow cost.

INDUSTRIAL APPLICABILITY

Another object of the invention relates to the use of the new tooldescribed in detail above, especially as a piercing plug, forgingmandrel or rolling mandrel for the production of seamless tubes orhot-forging tubular workpieces of metal.

EXAMPLES Example 1

On the surface of a piercing plug a mesh preformed by forming the shapeof the base steel was laid on with a steel wire thickness of 1.5 mm anda mesh width of 2.5 mm and welded. Successively, the composite has beenexposed to a thermo-chemical oxidation. A coherent continuous oxidelayer of 2500 microns thickness has been obtained.

FIG. 1 shows a hot forming tool in the form of a piercing plug in a sideview. The tool 1 has a tool body 2 having a work area 3, which extendsin the direction of an axis A over a certain length. In the work area 3,the tool is provided with a coating 4 which protects the tool 1 againstthermal and mechanical stress.

FIGS. 2 and 3 show the detail “Z” in the horizontal section through thetool according to FIG. 1 once for the material body with raised reliefbefore and after the production of the oxide protective layer(“scaling”).

In FIG. 2a one recognizes the saw-shaped relief which is formedaccording to Example 1 by applying a wire mesh. In this case, the basebody is characterized by the reference numeral 6, the mesh by thenumeral 7. In FIG. 2b it is seen that a part of the surface of therelief has been converted to oxide, but also between the loops of themesh, the surface of the base body has been oxidized (hatching withreference numeral 8).

The FIGS. 3a and 3b are analogue, however, the relief here has nosquare, but a round cross section. Again, one can see that the oxidelayer (hatching) is developing in equal proportions above and beneaththe original surface of the ferrous body.

1. A hot forming tool comprising a tool body having at least pro ratasurface coating, obtainable in that the basic body is provided with araised metallic relief, which is subsequently completely or partiallyoxidized and converted into a protective layer.
 2. The tool according toclaim 1, wherein it is a piercing plug, a forging mandrel or a rollingmandrel.
 3. The tool according to claim 1, wherein the basic body ismade of metal, preferably steel.
 4. The tool according to claim 1,wherein the raised relief on the base body is a wire wrapping.
 5. Thetool according to claim 1, wherein the raised relief on the base body isa metal fabric.
 6. A method for producing a hot forming tool comprisinga tool body having at least pro rata surface coating, in which (a) thebody is provided with a raised metallic relief, and (b) successively themetallic relief is completely or partially oxidized and converted into aprotective layer.
 7. The method according to claim 6, wherein the raisedrelief is applied on the base body by winding a wire.
 8. The methodaccording to claim 6, wherein the raised relief is applied on the basebody by covering with a metal fabric.
 9. The method according to claim8, wherein the metal fabric is preformed by deformation on the shape ofthe tool and then put onto the basic body.
 10. The method according toclaim 7, wherein the wire or the metal fabric is welded to the basebody.
 11. The method of claim 6, wherein the raised relief is applied onthe base body by Chemical/Physical Vapor Phase Deposition.
 12. Themethod according to claim 6, wherein the raised relief is applied on thebase body by thermal spraying.
 13. The method according to claim 6,wherein the base body consists of metal, preferably steel, and thematerial forming the raised relief, is capable of at least partlyforming an oxide layer.
 14. The method according to claim 6, comprisingcomplete or partial conversion of the metallic reliefs in an oxidicprotective layer by flame spraying, plasma spraying, or Is carried outby a thermochemical process.
 15. A method for the manufacture ofseamless tubes or hot-forging tubular workpieces of metal, comprisingusing the tool according to claim 1.